The semiconductor industry currently uses different types of semiconductor-based imagers, such as charge coupled devices (CCDs), complementary metal oxide semiconductor (CMOS) devices, photodiode arrays, charge injection devices and hybrid focal plane arrays, among others.
Because of the inherent limitations in CCD technology, CMOS imagers are being increasingly used as low cost imaging devices. A fully compatible CMOS sensor technology enabling a higher level of integration of an image array with associated processing circuits would be beneficial to many digital applications such as, for example, in cameras, scanners, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems and data compression systems for high-definition television.
A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, or a photodiode for accumulating photo-generated charge in a portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output transistor, which receives photogenerated charges from a doped diffusion region and produces an output signal which is periodically read out through a pixel access transistor. The imager may optionally include a transistor for transferring charge from the photoconversion device to the diffusion region or the diffusion region may be directly connected to or part of the photoconversion device. A transistor is also typically provided for resetting the diffusion region to a predetermined charge level before it receives the photoconverted charges.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the floating diffusion node accompanied by charge amplification; (4) resetting the floating diffusion node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge. Photo charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion node. The charge at the floating diffusion node is typically converted to a pixel output voltage by a source follower output transistor.
Each pixel cell receives light focused through one or more micro-lenses. Micro-lenses on a CMOS imager help increase optical efficiency and reduce cross talk between pixel cells. A reduction of the size of the pixel cells allows for a greater number of pixel cells to be arranged in a specific pixel cell array, thereby increasing the resolution of the array. In one process for forming micro-lenses, the radius of each micro-lens is correlated to the size of the pixel cell. Thus, as the pixel cells decrease in size, the radius of each micro-lens also decreases.
For example, as shown in FIGS. 1 and 2D, a portion of a CMOS imager 10 is shown. FIG. 1 illustrates an array of pixel cells for the CMOS imager 10, while FIG. 2D illustrates a single pixel cell. The illustrated CMOS imager 10 has a micro-lens 24 mounted on a frame 22, a color filter 20, a mask 16, and a pixel cell 12 with a photoconversion device 14. Electromagnetic radiation, such as light 26, is collected by the micro-lenses 24 and transmitted through the gaps 18 in the mask 16 toward the photoconversion devices 14. With a reduction in the size of photoconversion devices 14, and hence the pixel cells 12, and no consequential change in the thicknesses of any of the frame 22, color filter 20, and/or mask 16, the reduced radius of each micro-lens 24 causes the light 26 to focus at a focal point F1 above the respective photoconversion devices 14, thus causing a reduction of the amount of light received in the photoconversion devices 14 and an increase in cross talk between pixel cells 12.
It is desirable to increase the amount of light received by the photoconversion devices 14. One way to ensure that additional light is received by the photoconversion devices 14 is to increase the amount of light collected by the micro-lenses. One undesirable aspect encountered in conventional processes for forming micro-lenses is that the micro-lenses are subject to a size reduction. Specifically, one step in a conventional micro-lens formation process is to reflow an intermediate lens structure, which causes the intermediate lens structure to retreat, or pull back, at the corners and/or edges.
Referring specifically to FIGS. 2A-2C, 5A and 5B, intermediate lens structures 30 are formed of a lens material 34 on a wafer 32. The lens material 34 is preferably a transparent photosensitive polymer spin coated onto the wafer 32. Then, each intermediate structure 30 is exposed in a lithographic stepper with an image of the desired raw lens shape. Unwanted lens material is then removed from the intermediate lens structures 30 (such as around the periphery of each lens structure 30) in a wet developer. If the lens material 34 is formed of a positive resist, the exposed resist dissolves in the developer. If the lens material 34 is formed of a negative resist, the unexposed resist dissolves in the developer. Then, the intermediate lens structures 30 are reflowed to form the micro-lenses 24, each having a radius R1. The radii of the thus formed micro-lenses can be influenced only slightly by changing reflow bake conditions, the thickness and/or the composition of the lens material 34.
The intermediate lens structure 30, which was initially rectangular in the X-Y, X-Z, and Y-Z planes (FIGS. 2A, 2B and 5A), was transformed into a micro-lens 24 that is rounded in the X-Z plane (FIG. 2C). However, with specific reference to FIG. 5B, the micro-lens 24 formed in the reflow also is rounded in the X-Y plane. Specifically, the corners and edges of the intermediate lens structures 30 are pulled back from each other. This pull-back reduces the surface area of the micro-lenses 24, which will lead to a decrease in the amount of light collectable in, and hence transmittable from, the micro-lenses 24.